The present invention relates to a method for estimating satellite thruster uncertainty.
For efficient propellant/fuel optimization, many satellites use bipropellant thrusters for all satellite maneuvering modes. For a precision pointing mission requirement, the knowledge of the minimum I.sub.bit pulse width of a thruster is essential to limit attitude control errors. For a zero bias momentum three-axis stabilized attitude control system with highly efficient bipropellant thrusters, precision pointing is often limited by the I.sub.bit uncertainty during all thruster modes.
The repeatability of bipropellant thrusters of a satellite with regard to the minimum I.sub.bit is not well characterized over an entire satellite mission life. This lack of knowledge of the thruster I.sub.bit is manifested in the inherent uncertainties in the thruster on-time with each thruster firing. Many factors contribute to these uncertainties and include thruster duty cycle time, inlet pressure, and inlet temperature. Because of the uncertainty of I.sub.bit over the life of the satellite mission, precision pointing with a bipropellant thruster is very difficult. This is particularly true in the transition from stationkeeping mode, which is used to place a craft in its orbital slot, and normal mode, which is the mode for approximately 95% of the life of a functional satellite.
Most satellites require nutation damping during such a transition, using either hardware or software. Typical hardware used for nutation damping includes gimballed momentum wheels, control moment gyroscopes, reaction wheels, or magnetic coils.
A technique for the stabilization of a satellite is disclosed in U.S. Pat. No. 4,537,375 to Chan. The Chan patent discloses the use of counteracting thrusters with thruster level mismatch and misalignments. The level mismatch and misalignments are eliminated by pre-bias of pulse width modulation of pulse-width/pulse-frequency modulation bursts of thrusters during maneuvering. Chan recognizes the difficulty in calibrating induced attitude error to burst because of controller compensation delays, and seeks a way to avoid such calibration using a multi-thruster approach. The Chan system fails to eliminate the need for nutation damping devices.